She is not considered an ordained minister, but all employees are considered lay ministers and are expected to demonstrate, or at least not contradict, the tenets of the Catholic faith while at work. Her homosexuality was not an issue if she kept it private and did not hold it out as an acceptable lifestyle. The minute she became public with it, she was directly contradicting church teaching and thus was failing in her lay ministerial duties.
If she got “married” and never told her class or made a school public announcement, would the school have fired her? I guess she would have to tell the school for health insurance reasons.